This invention relates to a method and an apparatus for determining heat flow through a substrate. More particularly, this invention relates to an apparatus for field calibration of heat flow sensors and to a method for accurately measuring heat flow in the field utilizing field calibrated heat flow sensors.
Reliable quantitative thermal performance (heat flow) measurements of existing building and roofdeck systems while exposed to conditions of actual use have heretofore proven difficult to obtain. Information from such measurements, if available, could be used to compare actual in-place performance with theoretically expected performance, e.g., of insulation systems and materials. In-place measurement would also provide information on peak energy loads and average energy losses for the particular system under the actual conditions of use. Improvements in the design and construction of building and roofdeck systems could materialize from such information.
Heat flow measurements have been previously made on various substrates such as walls and roofdecks utilizing commercially available heat flow sensors. These sensors are generally in the form of thin disks and comprise a very dense thermopile system capable of outputting a voltage signal related to an average temperature differential across the thickness of the disk. This voltage signal is converted to a quantitative heat flux, e.g., BTU/h.ft.sup.2, on the basis of a calibration parameter for the sensors.
Generally, heat flow measurements with such sensors have been made with the sensor embedded in the substrate such that heat transfer is primarily, if not exclusively, by the conductive mode. Calibration of the sensor has heretofore been carried out under controlled, laboratory conditions. The calibration is typically conducted in a guarded hot plate or heat flow meter apparatus. The sensor is mounted on or embedded in a substrate mounted in the apparatus and calibrated by measurement of the voltage generated by the sensor in response to a known input of heat from a heat source within the apparatus. Calibration is normally conducted in accordance with ASTM-C177 in the guarded hot plate or C518 in the heat flow meter apparatus and may provide a satisfactory calibration parameter for use of the sensor when embedded in an experimental substrate since heat transfer under the calibration conditions is by conduction.
For purposes of providing reliable on site heat flow measurements, however, the practice of embedding the heat flow sensor in an experimental substrate, e.g., a wall of a room or a roofdeck, may not be satisfactory or practical. At least localized destruction of the substrate is required and the measurements obtained are unlikely to be representative of the actual substrate.